This invention relates to a method for separating and recovering elemental sulfur (hereunder referred to simply as xe2x80x9csulfurxe2x80x9d) from minerals and other sulfur-containing (sulfurous) compounds, particularly from the residues obtained in the leach step in a hydrometallurgical zinc production process, wherein zinc is leached from a zinc concentrate which is concentrated zinc sulfide.
Steady efforts are being made to improve upon the zinc production technology. A particularly important proposal can be found in Japanese Patent No. 2856933 under the title xe2x80x9cA Wet Metallurgical Method for Treating Zinc Sulfide-Containing Raw Concentratexe2x80x9d. According to this method, zinc present in a zinc concentrate in an amount stoichiometrically equal to the iron content in a calcine, the product of oxidizing roasting of the concentrate, is directly leached from the concentrate and recovered as soluble zinc, thereby increasing the yield of zinc production. This is an excellent approach in that the yield of electrolytic zinc production can be increased without enhancing the capacities of the existing oxidative roasting furnaces and sulfuric acid production facilities.
For industrial practice, however, the concentration of ferric ions which are indispensable for direct leaching of zinc sulfide in zinc concentrates is low and, hence, the yield of direct leaching from the zinc concentrate is too much limited to realize a significant increase in the throughput of zinc production. With a view to solving this problem, the present inventors developed a method in which the residue from neutral leaching and the iron precipitate obtained in the hydrometallurgical zinc production process was repulped and leached to realize a significant increase in the yield of direct leaching from the zinc concentrate, thereby achieving a significant increase in the yield of electrolytic zinc production without enhancing the capacities of the existing oxidizing roasting furnaces and sulfuric acid production facilities (commonly assigned Japanese Patent Application No. 021143/2000).
A problem with this method concerns the two different forms of sulfur that occur in zinc concentrates, one being as a metal sulfide and the other in the free state. As the yield of direct leaching from the zinc concentrate increases, there occurs a further increase in the generation of free sulfur in the leaching step; this has presented the need to develop a more efficient way to recover the generated sulfur from the residue from the leaching step.
The following two methods have conventionally been used to separate and recover sulfur from sulfurous compounds:
(1) the residue from the leaching of a sulfide ore is subjected to flotation and the resulting sulfur-rich xe2x80x9cfloatxe2x80x9d (sulfur concentrate) is heated to a temperature higher than the sulfur""s melting point (119xc2x0 C.) in order to melt sulfur; and the molten xe2x80x9cfloatxe2x80x9d is then filtered to separate and recover the molten sulfur;
(2) a sulfurous compound such as the xe2x80x9cfloatxe2x80x9d is heated to a temperature higher than the sulfur""s boiling point (445xc2x0 C.), preferably between 450 and 500xc2x0 C., so that sulfur is distilled for separation and recovery.
However, these methods are not industrially applicable for sulfur recovery in the zinc producing method proposed in Japanese Patent Application No. 021143/2000. In the first method in which the sulfur in the xe2x80x9cfloatxe2x80x9d is melted and then recovered by filtering, the impurities in the xe2x80x9cfloatxe2x80x9d are most likely to contaminate the molten sulfur in the process of filtering and the sulfur that can be recovered contains more impurities than the sulfur recovered from the ordinary oil refining process and need be rendered to have an adequately higher purity by a subsequent refining step. What is more, if the temperature control is not appropriate, the once melted sulfur returns to a solid state, increasing the frequency of operating troubles such as a failure to filter the molten xe2x80x9cfloatxe2x80x9d. As a further problem, sulfur which has a low chemical reactivity at ordinary temperatures becomes highly reactive at elevated temperatures and forms sulfides with almost all metals except gold and platinum, as well as combines with many non-metallic elements. Hence, filtering the heated xe2x80x9cfloatxe2x80x9d and recovering the molten sulfur requires increased operating cost inclusive of plant maintenance cost.
In the second method, considerable thermal energy is required to heat the xe2x80x9cfloatxe2x80x9d or other sulfurous compounds to the temperature of 450-500xc2x0 C. and the operating cost is even higher than in the first method, discouraging the zinc smelter from industrial practice of the second method. What is more, the temperature to which the sulfurous compound is heated is closer to 630xc2x0 C. (the catch fire point of sulfur) than is the temperature at which sulfur is heated to melt in the first method. This has posed the need to develop a safer method of sulfur recovery.
The present invention has been accomplished under these circumstances and has as an object providing an efficient and low-cost method of recovering high-purity sulfur from sulfurous compounds such as the sulfurous residues from the step of leaching zinc concentrates.
The present inventors made intensive studies in order to attain this object. As a result, they found that quite contrary to the common sense underlying the previous practice of heating the xe2x80x9cfloatxe2x80x9d or other sulfurous compounds to a temperature higher than the sulfur""s boiling point (445xc2x0 C.) in order to achieve evaporative separation of sulfur, sulfur had an unexpectedly high vapor pressure even in the liquid phase at temperatures near its melting point. The inventors continued their studies on the basis of this finding and finally accomplished the present invention by which high-purity sulfur can be efficiently recovered from sulfurous compounds such as the residues from the leaching of zinc concentrates at temperatures below the boiling point of sulfur.
The stated object of the invention can be attained by a method of recovering sulfur from a sulfurous compound by the steps of heating the sulfurous compound to a temperature not lower than the melting point of sulfur but less than its boiling point and cooling the evolving gas containing sulfur vapor to a temperature less than the melting point of sulfur so as to condense the sulfur vapor.
Preferably, the sulfurous compound is a sulfur-rich xe2x80x9cfloatxe2x80x9d obtained by flotation of the sulfurous residue from the step of leaching a zinc concentrate in a hydrometallurgical zinc production process.
Preferably, said hydrometallurgical zinc production process comprises the steps of:
(1) (neutral leach step) subjecting calcine (zinc oxide) to neutral leaching with a return acid containing free sulfuric acid from the electrowinning step in the zinc production system to produce a neutral leach liquor (zinc sulfate solution), said calcine being obtained by oxidizing roasting of part of a zinc concentrate;
(2) (zinc concentrate leach step) repulping the zinc ferrite containing residue from the first step with the return acid (spent electrolyte) from the electrowinning step in said zinc production system, adding a zinc concentrate to the resulting pulp and leaching the zinc sulfide in the added zinc concentrate at a temperature between 90xc2x0 C. and the boiling point of the pulp at atmospheric pressure;
(3) (iron oxidizing step) adding an oxidizer to the leach liquor from the second step to oxidize the ferrous ions in the leach liquor to ferric ions and adding a zinc oxide based calcine to the leach liquor as a neutralizer, thereby recovering the iron in the leach liquor as an iron precipitate., and supplying at least part of the iron precipitate as an iron source to the second step while recycling the neutralized liquor to the first step;
(4) (residue treating step) treating the iron precipitate from the third step; and
(5) (flotation step) subjecting the sulfurous residue from the second step to flotation.